1 6 THE PLANT CELL 



gives rigidity and strength to, and preserves the form of, the 

 whole plant body. When first formed the wall is usually 

 cellulose niC^K^^O^); we know, however, that we are including 

 under this name a group of closely allied substances which yield 

 different products on chemical decomposition. 



The wall may remain cellulose throughout the existence of 

 the plant, or it may become modified in different tissues to meet 

 special requirements. Thus, in the bast and wood fibers, 

 the walls become lignified; and in the epidermis and cork they 

 are cutinized and suberized. It is not known precisely what 

 the chemical nature of these modifications is, but lignification 

 seems to increase the hardness, strength, and elasticity with- 

 out decreasing the permeability of the walls; while cutiniza- 

 tion and suberization involve infiltration with waxes, so that the 

 walls are made more or less impermeable to water and gases, as 

 happens to paper when infiltrated with melted paraffin. Some- 

 times the wall undergoes a mucilaginous modification by which, 

 if the modification is extensive, it swells enormously on coming 

 in contact with water. Walls of this kind occur at the surface 

 of many seeds, such as flax and mustard, where they are useful 

 in gluing the seeds to the substratum; and they are frequently 

 foimd in desert plants, where they are useful in imbibing and 

 holding stores of water. 



The cell- wall when first formed is relatively very thin, and 

 its growth in thickness and extent is accomplished by the addi- 

 tion of new particles within it (growth by intussusception) and at 

 its surface (growth by apposition). On account of the unequal 

 swelling of the cell-Walls in water in its different dimensions, and 

 its behavior like a crystal in polarized hght, Nageli has con- 

 ceived the hypothesis that it is composed of aggregations in 

 crystalline form of minute parts or molecules to which he has 

 given the name micellcB. He conceives that the micellae are 

 separated by films of water which become thicker on the swelling 

 and thinner on the drying up of the wall. This hypothesis still 

 explains better than any other the optical properties of the cell- 

 wall and the phenomena of imbibition. 



